Sterilization is the absolute destruction of any virus, bacteria, fungus or other microorganism, whether in a vegetative or in a dormant spore state. Conventional sterile processing procedures for medical instruments involve high temperature (such as steam and dry heat units) or toxic chemicals (such as ethylene oxide gas, EtO). Steam pressure sterilization has been the time-honoured method of sterilization. It is fast and cost effective. However, the autoclave destroys heat-sensitive instruments. Thus, since more and more heat-sensitive instruments such as arthroscopes and endoscopes are used in medical treatment, other types of sterilization need to be used.
Ethylene oxide sterilization is used to cold sterilize heat-sensitive instruments. Until recently, ethylene oxide sterilization was the state of the art method for cold sterilization. Ethylene oxide sterilizes heat and moisture-sensitive objects and penetrates very well. However, it has been deemed by national health and safety organizations to be carcinogenic and neurotoxic. Additionally, since it is a highly flammable gas, it is normally combined with CFCs (chlorofluorocarbons) for safety reasons. However, due to the deleterious effects of CFCs on the ozone layer, their use has been banned by the Montreal protocol in 1996. Moreover, ethylene oxide requires long sterilization and aeration periods, since the molecule clings to the surface of instruments. The total sterilization time is 14 to 36 hours depending upon the materials to be sterilized. This type of sterilization necessitates the use of containment rooms, monitoring systems, and room ventilators.
A more efficient, safer, and less expensive sterilization agent was needed and has been found in the form of ozone O3 which is the fourth most powerful, but overall most desirable oxidizing agent (the three more powerful agents being fluorine derivatives which are too unstable and toxic for safe use in sterilization). Ozone can easily be generated from oxygen, especially hospital grade oxygen. Oxygen is readily available in the hospital environment, usually from a wall or ceiling oxygen source, or, if mobility is required, from a portable “J” cylinder of oxygen.
Ozone is widely used in industry as oxidizing agent to bleach paper pulp, treat drinking water, and sterilize sewage water and food products. Ozone generally acts on chemical compounds in two ways. Either by direct reaction or through hydroxyl radical species formed during the decomposition of ozone (Encyclopaedia Of Chemical Technology, Vol. 17, Ozone page 953 to 964). The amounts (concentrations) of ozone required in the sterilization gas for water purification are low, generally less than 36 mg/l (milligram per liter). However, significantly higher concentrations are required to make ozone gas an effective sterilant of micro-organisms, those high concentrations of ozone gas have to be combined with critical levels of humidity during the entire sterilization cycle. The activity of ozone increases rapidly with increased relative humidity. The resistance of spores to ozone varies from strain to strain, but the differences become comparatively small at high relative humidity (Ishizaki et al., 1986. Inactivation of the Silas spores by gaseous ozone, J. Appl. Bacterial, 60:67–72). A high relative humidity is required for the ozone to penetrate the protective shells of microorganisms. The presence of water often accelerates ozone reactions with organic substances (Langlais et al., (EDS), 1991, Ozone in Water Treatment, Application and Engineering. Louis Publishers: Chelsea, Mich., 569 pages). Sufficient relative humidity is also required in order to enable ozone to penetrate the normally used sterilization packaging. Thus, it is desirable to humidify this ozone gas used for sterilization applications.
Various ways of humidifying ozone-containing gas used for sterilization treatments are known in the field of ozone sterilizers.
The use of a mixture of ozone gas with a very fine water mist in a sealed plastic bag container, which contains an article to be sterilized, is described in U.S. Pat. No. 3,719,017. The method disclosed involves repeated evacuation and refilling of the plastic bag with a mixture of ozone gas and a very fine water mist. The air in the bag is exhausted and replaced with a pressurized mixture of ozone and water mist. Upon encountering the much lower pressure within the bag, the water particles from the pressurized mixture explode, forming a water mist. However, this system cannot generate a sufficiently high water vapour concentration to provide and maintain the required high relative humidity.
A review of more recent patents shows that the relative humidity required for successful sterilization is at least 85% throughout the process. U.S. Pat. No. 5,069,880 describes a device capable of generating such a high relative humidity. In the apparatus described, the ozone gas is bubbled through a water bath in an effort to increase the water content of the gas. Although ozone at 85% humidity can kill most micro-organisms, it does not meet the “worst case scenario” stipulated in North American standards. Moreover, the device described is unable to generate humidity levels higher than 85%.
North American standards set by agencies such as the Food and Drug Administration and Heath Canada require sterilizer manufacturers to meet worst-case scenario requirements. A sterilization gas including 85% humidity is insufficient for achieving the targeted results. A minimum relative humidity level of 95% is required to meet the standards imposed.
Water evaporates at 100° C. at atmospheric pressure (1013 mbar). Thus, various prior patents (see Faddis et al., U.S. Pat. Nos. 5,266,275; 5,334,355; and 5,334,622) teach sterilization systems wherein water is heated to above the boiling point to produce steam for injection into the ozone-containing gas produced by an ozone generator. The steam is heated to 120° C. Thus, the vapour/ozone mixture used for sterilization presumably has a temperature close to 100° C. However, since the decomposition of ozone increases exponentially with temperature in the range of 20 to 300° C., injecting the water vapour at a temperature of about 120° C. leads to premature ozone decomposition. As a result, the effective ozone concentration in the gas produced by the ozone generator is reduced, thereby requiring significantly increased treatment times and the generation of much larger amounts of ozone gas for each sterilization cycle. On the other hand, if the temperature in the sterilization chamber is not maintained at above the boiling temperature of water, condensation will occur. However, a layer of condensation on any article to be sterilized will significantly reduce the effectiveness of the ozone sterilization process, if not completely block sterilization of the covered area. This problem should be avoided, but is not recognized in the art.
Moreover, carrying out the sterilization at an elevated temperature and close to 100° C. will require a substantial cooling down period for the sterilized materials, thereby making the sterilization a lengthy and inefficient process. Thus, a more efficient and effective sterilization method and apparatus is desired for the sterilization with ozone at a relative humidity above at least 95%.